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Because a DDS is a Nyquist device, the DAC output spectrum has Nyquist images. The result is that the DAC harmonics can appear at unexpected frequency locations. This becomes readily apparent when a FTW sets the DDS output frequency close to an integer sub-multiple of the system clock, like one-fourth, for example. Shown here is a spectral plot of the DAC output from a DDS with a 100 mega-hertz system clock and the DDS tuned to an output frequency of 25.153 MHz (approximately one-fourth of the system clock frequency). Normally, one would expect the third harmonic to appear at 75.459 MHz, or 3 times the 25.153 MHz fundamental frequency. However, this puts the third harmonic beyond the Nyquist frequency of 50 MHz. The result is that the third harmonic gets shifted to the system clock frequency minus its expected frequency, or 24.541 MHz, which appears 612 kHz to the left of the fundamental. This diagram demonstrates how DAC harmonics tend to relocate near the fundamental when the DDS is tuned near an integer sub-multiple of the system clock. In fact, when a DDS is tuned to an exact integer sub-multiple of the system clock frequency, some of the DAC harmonics happen to coincide with the fundamental and seem to magically disappear from the output spectrum.
